Hello, I have a question that i'd like to ask, but the wording is a little tricky, so I hope you can get the gist of what i'm trying to describe. I was thinking about the thought experiment that is often used to describe relative motion - a light clock on a train passing through a station being observed from the platform. From there I had an idea that isn't directly related, but it came out of the train. Imagine you have a train travelling along a straight piece of track and on board there is a photon emitter aiming perpendicular to the direction of travel. (This is where I don't think I explain it too well). Before the photons have been emitted, can they be said to exist? I ask this because i'm wondering whether they already have momentum in direction the train is travelling in when they are emitted. So, would an emitted photon leave the train perpendicular to the direction of travel, or at some angle caused by a component in the direction of trains motion? I hope that makes sense and that this is in the correct part of the forum, Thanks in advance, Lntz.